What is the correct setting for max_client_conn using session pool mode in pgBouncer?

Question

Without diving so much into web applications or database performance tuning etc, I wish to consult on one thing. pgBouncer has the following settings:

;; Total number of clients that can connect
max_client_conn = 5000

;; Default pool size.  20 is good number when transaction pooling
;; is in use, in session pooling it needs to be the number of
;; max clients you want to handle at any moment
default_pool_size = 500

Given that the web application I am running can only operate in session mode, is the configuration above correct assuming:

1. My web application does not go above the 5000 client connection mark
2. My PostgreSQL Database has a max_connection setting of 500

Or am I missing something here. Thanks!!

Answer 1

You will probably get errors about too many open files long before you reach 5000 queued connections, and so need to change the limit using ulimit -n or by otherwise altering the kernel settings. (Note that if your application needs max_client_conn to be that high, your application is almost certainly profoundly broken. But that wasn't your question)

You don't want default_pool_size to be equal to max_connections. That would leave no connections left over for things like monitoring, reporting, or debugging, or anything else. So back off a few. If 495 isn't enough, 500 probably wouldn't be enough either.

Note that 500 is unlikely to be a good setting of max_connections in the first place. Do you have 500 CPUs, or a RAID with 500 spindles? But again, that wasn't your question.